Free Upgrade to Windows 10 after The Public Deadline
Free upgrade is officially for customers who use assistive technologies, but at the moment apparently works for everyone.
Microsoft FAQ: "We have not announced an end date of the free upgrade offer for customers using assistive technology. We will make a public announcement prior to ending the offer."
However, as pointed out by wintoand, Microsoft is planning to block it in the coming weeks and make it available only with Microsoft support: